Hypoglycemia is a major barrier to the achievement of adequate glycemic control for most patients with insulin- dependent diabetes, both those with type 1 diabetes and advanced type 2 diabetes. Type 1 diabetic patients with absolute insulin deficiency (C-peptide negative) are at greatest risk for experiencing severe hypoglycemic events because the near total destruction of insulin producing islet beta-cells produces an associated defect in glucagon secretion from neighboring alpha-cells. Such patients then depend on the sympathoadrenal system as a final defense against hypoglycemia, but unfortunately, recurrent episodes of hypoglycemia blunt sympathoadrenal activation and produce a syndrome of hypoglycemia unawareness that is associated with a twenty-fold increased risk of life-threatening hypoglycemia. Without intact islet or sympathoadrenal (especially epinephrine) responses to hypoglycemia, these patients cannot increase endogenous (primarily hepatic) glucose production to prevent or correct low blood glucose. In the present application we propose to determine whether strict hypoglycemia avoidance by 2 novel therapeutic approaches for type 1 diabetes, namely islet cell transplantation (specific aim 1) or real-time continuous glucose monitoring (RT-CGM; specific aim 2), can restore endogenous glucose production in response to hypoglycemia in patients with long standing disease. Under specific aim 1, 12 subjects with long standing type 1 diabetes complicated by hypoglycemia unawareness will undergo assessment of the endogenous glucose production response to insulin-induced hypoglycemia using paired hyperinsulinemic eu- and hypoglycemic clamps with stable glucose isotope infusions before and at 6 and 18 months following islet cell transplantation. Under specific aim 2, 12 similar type 1 diabetic subjects with hypoglycemia unawareness will undergo identical assessment of the endogenous glucose production response to insulin-induced hypoglycemia before and at 6 and 18 months following initiation of RT-CGM. Because islet transplant recipients may require some insulin to control hyperglycemia, and because RT-CGM may be interrupted or fail to arouse a sleeping patient, it is critical to understand what improvements in glucose counterregulation may be offered by either approach. While some patients may only be candidates for only one approach or the other, if both approaches are shown to restore glucose counterregulation, the data generated from this proposal will enable the design of future randomized trials of cell vs. mechanical therapy on long-term glucose counterregulatory responses and the protection thus offered against severe hypoglycemia.